Thanks to “anonymous” for the tips on keeping spammers out of the Teen Challenge forum.
I’ve found that the easier way to deal with spammers in phpbb involves changing the signup page so they (a) can’t find it in google or (b) can’t use it properly. The best hack I’ve heard of changed the page to include an extra “website” field that wasn’t visible to regular users (but would be filled out by bots) and then any user who filled it out was automatically rejected…
Unfortunately if Google can’t find the signup page then no one else (legitimate users) can either so I can’t use that one but I am definitely interested in the second idea. I’ll have a look at the phpBB mods and see what I can find.